In phosphate rock sources there are naturally occurring fluoride contaminants that dissolve with the phosphate materials during the reaction of the phosphate rock with an acid. In most of the phosphoric acid processes, sulfuric acid is used to acidulate the rock, which results in the production of phosphoric acid and by-product gypsum (calcium sulfate dihydrate, with the chemical formula CaSO4.2H2O). This is generally referred to as the “wet” phosphoric acid process.
In the acidulation stage, as the rock is reacted with the sulfuric acid, the fluoride contained in the rock also dissolves into the acid phase. Once the fluoride is in solution it has a relatively high vapor pressure, and a portion of the fluoride will evolve from the acid/gypsum slurry in the form of a silico-fluoride vapor, primarily silicon tetrafluoride (SiF4) and/or hydrofluosilicic acid (H2SiF6), along with some hydrofluoric acid (HF). Also during the reaction of phosphate rock and sulfuric acid there is heat generated as a result of the reaction.
To maintain the temperature in the reaction system many plants typically use a method referred to as flash cooling to remove heat from the phosphoric acid/gypsum. In this approach the slurry is pumped into a vessel that is under vacuum, and at this reduced pressure the slurry will begin to boil and evaporate water plus other vapors from the slurry, which in turn results in the cooling of the slurry.
The vacuum for the cooler is generally supplied via the use of a so-called barometric condenser, where water (typically from a recirculating cooling pond system) is pumped into a spray-tower-like vessel and the condensing of the water vapor exiting the flash cooler generates a vacuum. This technique is well established in the industry, and thus results in fluoride species from the plant generally ending up in, and contaminating, the associated pond wastewater systems.
In many facilities, a large cooling pond is used to provide the water for the various barometric condensers that are used within the phosphoric acid process. The water leaving the barometric condensers is now warm and the use of a large area cooling pond allows for the cooling of the used water, primarily via evaporation of some of the water in the ponds, and the eventual recycle of the cooled water back to the barometric condensers.
As a result of the fluoride species vapor pressure in the slurry, a portion of the fluoride species will evolve with the water vapor in the flash cooler and combine with the water used to provide vacuum for the cooler. The used barometric condenser water will then begin to accumulate the fluoride species, along with other contaminants such as entrained phosphoric acid. Over a period of time, the fluoride species concentration in the pond water will tend to build up to somewhat of a pseudo-steady-state value.
In most operations, the phosphoric acid produced in the digestion system is evaporated to produce a higher concentration material for use in subsequent fertilizer manufacture or production of merchant grade acid (MGA). These systems are generally operated under vacuum which is supplied via direct contact barometric condensers. As in the flash cooling case, fluoride vapors are evolved during the evaporation step and are collected in the recirculated water used for the condenser. The cool water supplying the barometric condenser heats up as it condenses both water vapor and the accompanying fluoride vapors. This warm water is returned to the pond system for cooling.
Additional fluoride vapors are also emitted from other sections of the process and are typically recovered via the use of a scrubber system, where the vapors are contacted with pond water to “scrub” the contaminants from the vapor stream. Typically, the same water that is used for the barometric condensers in the plant is used for the various scrubbers used for general emissions.
After production of the phosphoric acid, the mixture of gypsum (CaSO4.2H2O) solids and phosphoric acid (P2O5) solution is filtered to separate the phosphoric acid from the gypsum. The gypsum is then washed with water, typically from the pond water system, and the wash carried out in a counter-current fashion, with the resulting phosphoric acid (P2O5)-enriched water added into the phosphate rock/sulfuric acid reaction or digestion circuit. Even though the counter-current washing is efficient, there is still some phosphoric acid lost as a result of entrainment of dilute solution in the moist gypsum cake solids. While the amount is generally small, with the scale of modern phosphoric acid plants, this small loss can still represent an appreciable cost to the operation.
The gypsum by-product is generally discharged into so-called “gypsum ponds”. These ponds are relative large since there are roughly 5.5 tons of gypsum produced for every ton of phosphoric acid (P2O5) produced. Also, as the gypsum “stacks” up in the pond it also entrains some of the contaminated water within the gypsum material itself and acts as somewhat of a storage pile for soluble fluoride species and dilute phosphoric acid that accumulate in the pond water. As a result of this “stacking” effect, the gypsum stack acts as an accumulator of the various contaminants, e.g. fluoride species, dissolved phosphoric acid (P2O5), etc. Due to the ratio of the gypsum produced to phosphoric acid (P2O5) produced, and the interaction of the gypsum and cooling ponds, the contaminants will tend to build up to a pseudo steady-state value.
Once the phosphoric acid system has operated for a period of time, the amount of contaminants being stacked with the gypsum will closely approximate the net input of these contaminants into the pond systems. As this occurs, the pond water will exhibit somewhat of a constant contaminant concentration level. The gypsum is continuously acting as a surge reservoir for contaminated pond water and as a result can serve as an on-site inventory source.
Fluoride species can also evolve in other unit operations within the overall complex such as fertilizer production facilities. Again, recirculated pond water is generally used for scrubbing these vapors emitted by the other operations.
Since the majority of the fluoride vapors are evolved in the evaporation systems, there have been previous efforts to recover the fluoride via the installation of so-called fluosilicic acid (FSA) towers. These towers are essentially spray or other form of direct contact towers that are installed between the evaporator vapor discharge and the barometric condenser units. A recirculated stream of fluosilicic acid (FSA) is used to scrub the vapors from the evaporators and produce a more concentrated and higher purity stream of FSA.
Since the use of conventional FSA recovery involves the installation of additional equipment with the vacuum portion of the evaporation system, it can have a negative impact on the operation of the vacuum evaporator because it adds pressure drop into the circuit. Further, any up-sets in the evaporation system or efforts to operate at higher than designed flow rates, which are common in the industry during certain seasons, can result in off-specification FSA. Since FSA is a true by-product of the operation and not a primary product, any detrimental impacts of the FSA recovery operation on the phosphoric acid (P2O5) operation are generally viewed as negatives.
It would therefore be desirable to have a process which would allow for the recovery of fluorides from the phosphoric acid complex but have no potential negative impacts on the phos-acid operations. Further, in the past the FSA has generally been sold commercially as an FSA solution which has a relatively low unit fluoride value due to the presence of the silica component within the compound. Earlier efforts to separate the fluoride from the silica have used ammonia to precipitate the silica as an amorphous silica material and an intermediate ammonium fluoride solution. This ammonium fluoride can be further treated to produce various fluoride products, but in general the silica produced in the previous processes was of relatively low quality and was not competitive in the higher value industrial silica markets.
Due to the negatives associated with installing FSA recovery equipment within the evaporation system, the recovery of fluoride materials from phosphoric acid sources has been somewhat limited. Industrial economies require fluoride products for a variety of applications, and in many cases countries do not have domestic conventional fluoride sources such as fluorspar. This results in the need to import fluorides, and thus a dependency on a foreign fluoride source is created.
Therefore it would be desirable to have a process for the recovery of fluorides that has no negative impact on the existing phosphoric acid operation and allows for the production of high purity fluoride products as well as an industrially acceptable precipitated silica product. Further, it would be desirable for such a process to be able to recover fluoride species from the pond systems or barometric condenser waters, and which would also allow the phosphoric acid producer to recover not only the current fluoride being produced in the facility but material that had been previously stacked or inventoried in the gypsum and cooling pond systems.
A system that would recover not only the fluoride species from the pond waters but also a portion of the contained phosphoric acid (P2O5) that is in the water would also be desirable. In this manner the pond systems can now serve as a potentially value-added source of fluoride species and recovered P2O5 for the phosphoric acid operation.
Further such a system would reduce the costs associated with any wastewater treatment that might be required in the event of excess wastewater build-up in the pond systems. Treatment of the contaminated pond water is well established, but does represent a significant cost that is dependent on the concentration of the fluoride and P2O5 in the water. Reduction of contained contaminants prior to excess water treatment would significantly reduce the costs associated with water treatment.